Dual direction ESD protection produces challenges both in terms of size as well as triggering voltage.
The present applicant previously developed a dual direction clamp based on the use of a DIAC and ADIAC architecture. This solution has the advantage that it provides a small footprint dual direction device. However, since it is based on non-self aligned BJT junctions it does not always have a good turn on voltage. In fact with a turn on voltage of about 14V it is not suitable for applications requiring a low turn on voltage of about 6-8V.
In another prior art solution, the turn on voltage is addressed by making use of isolated cells of BJT, NMOS or BSCR devices that are packed back to back. This allows standard devices to be used and to make use of control electrode coupling to achieve low triggering voltage. In fact, by making use of silicon germanium (SiGe) BJTs suitable characteristics can be obtained. However, stacking BJTs back to back as proposed by this prior art solution, results in a large footprint device.